1. Field of the Invention
The present invention relates to a zoom lens suitable for an image pickup apparatus of a broadcast TV camera or a video camera.
2. Description of the Related Art
In recent years, the broadcast TV camera has been shifting to HDTV (high definition television), and image pickup systems that can realize higher definition images are demanded.
To meet the demand, image pickup elements with a higher definition and zoom lenses used as an image pickup optical system with improved performance (improved resolving power) have been developed.
An increase in the resolving power of the image pickup optical system enables resolution of high frequency components of objects.
However, this leads to a decrease in the focal depth of the image pickup optical system, and fine focus adjustment is required near the best focus position (or the best image plane).
In the case of manual focusing, a photographer performs focusing while viewing a relatively small image screen such as a view finder.
Therefore, highly accurate focusing has become difficult.
Under the above described circumstances, demands for zoom lenses having an auto focusing function (auto focus detection function) have been increasing.
Auto focusing systems include active auto focusing systems and passive auto focusing systems.
In the active auto focusing system, a separate ranging system is provided in addition to an image pickup optical system, and the distance of an object is measured by for example emitting infrared light from the ranging system toward the object and receiving the infrared light reflected by the object.
This positive auto focus system is not suitable for use in an image pickup apparatus of a TV camera, since operability and mobility are required to be ensured in the image pickup apparatus for a TV camera.
On the other hand, there is a type of passive auto focusing system in which a part of the lens units of the image pickup optical system or the image pickup element is driven along the optical axis direction by a small amplitude (i.e. wobbled) to obtain a signal for determining the direction of the best focus position (which system is sometimes referred to as a “a contrast detecting focus system”)
In this system, a part of the lens units of the image pickup system is moved minutely, which causes the problem that the movement of seeking the best focus position is reflected unsightly in the picked up image.
In another passive auto focus system, a part of light beams traveling in the optical path of an image pickup optical system for guiding light from an object to an image pickup element for picking up an image is branched, and the branched light beams are focused onto an image pickup element for focus detection to obtain a focusing signal.
Various such systems in which the focusing state is determined by a branching optical system (focusing optical system) different from the image pickup optical system have been known (see Japanese Patent Application Laid-Open No. H09-274129, Japanese Patent Application Laid-Open No. 2004-085676 and Japanese Patent Application Laid-Open No. 2003-270517).
These systems in which a branching optical system is provided in addition to the image pickup optical system have the advantage that it is possible to obtain a focusing signal while presenting clearer images without deteriorating the quality of picked up images.
The system in which a branching element for branching a part of beams is provided in the optical path of an image pickup optical system and the direction toward the best focus position is determined using a branching optical system different from the image pickup optical system has the advantage that the operation of seeking the focus position is not reflected in the picked up image.
However, the presence of the branching optical system added in the periphery of the image pickup optical system leads to an increase in the total length of the lens.
In addition, this also leads to an increase in the effective diameter of the lens, and the position of the exit pupil becomes close to the focus position, which causes the disadvantage that color shading occurs in the case of three-image-pickup-element TV cameras having a color splitting optical system.
In some zoom lenses for a TV camera, a part of the lens units in a relay optical system is inserted into/removed from the optical path in exchange for another lens unit (i.e. an extender lens) to change the focal length range of the entire system.
In that case, in detecting a best focus position signal by a light receiving element in the branching optical system, in the case where a light beam passing through the extender lens is incident on the branching optical system, the imaging magnification on the light receiving element changes.
In that case, the F number of the entire system changes depending on the insertion/removal of the extender lens, and the focal depth also changes therewith. Therefore, a high focus detection accuracy cannot be attained.
It is very important for image pickup optical systems (zoom lenses) for TV cameras or the like in recent years to satisfy requirements for down sizing and weight reduction of the image pickup optical systems, elongation of the exit pupil distance and excellent optical performance.
An object of the present invention is to provide a zoom lens that allows viewing of picked up images without being affected by insertion/removal of an extender lens into/from the optical path and at the same time enables highly accurate detection of a focus detection signal.